package org.apache.lucene.util.packed;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.apache.lucene.store.DataInput;
import org.apache.lucene.store.DataOutput;
import org.apache.lucene.util.CodecUtil;
import org.apache.lucene.util.Constants;

import java.io.IOException;

/**
 * Simplistic compression for array of unsigned long values.
 * Each value is >= 0 and <= a specified maximum value.  The
 * values are stored as packed ints, with each value
 * consuming a fixed number of bits.
 *
 * @lucene.internal
 */

public class PackedInts {

    private final static String CODEC_NAME = "PackedInts";
    private final static int VERSION_START = 0;
    private final static int VERSION_CURRENT = VERSION_START;

    /**
     * A read-only random access array of positive integers.
     * @lucene.internal
     */
    public static interface Reader {
        /**
         * @param index the position of the wanted value.
         * @return the value at the stated index.
         */
        long get(int index);

        /**
         * @return the number of bits used to store any given value.
         *         Note: This does not imply that memory usage is
         *         {@code bitsPerValue * #values} as implementations are free to
         *         use non-space-optimal packing of bits.
         */
        int getBitsPerValue();

        /**
         * @return the number of values.
         */
        int size();

        /**
         * Expert: if the bit-width of this reader matches one of
         * java's native types, returns the underlying array
         * (ie, byte[], short[], int[], long[]); else, returns
         * null.  Note that when accessing the array you must
         * upgrade the type (bitwise AND with all ones), to
         * interpret the full value as unsigned.  Ie,
         * bytes[idx]&0xFF, shorts[idx]&0xFFFF, etc.
         */
        Object getArray();

        /**
         * Returns true if this implementation is backed by a
         * native java array.
         *
         * @see #getArray
         */
        boolean hasArray();
    }

    /**
     * A packed integer array that can be modified.
     * @lucene.internal
     */
    public static interface Mutable extends Reader {
        /**
         * Set the value at the given index in the array.
         * @param index where the value should be positioned.
         * @param value a value conforming to the constraints set by the array.
         */
        void set(int index, long value);

        /**
         * Sets all values to 0.
         */

        void clear();
    }

    /**
     * A simple base for Readers that keeps track of valueCount and bitsPerValue.
     * @lucene.internal
     */
    public static abstract class ReaderImpl implements Reader {
        protected final int bitsPerValue;
        protected final int valueCount;

        protected ReaderImpl(int valueCount, int bitsPerValue) {
            this.bitsPerValue = bitsPerValue;
            assert bitsPerValue > 0 && bitsPerValue <= 64 : "bitsPerValue=" + bitsPerValue;
            this.valueCount = valueCount;
        }

        public int getBitsPerValue() {
            return bitsPerValue;
        }

        public int size() {
            return valueCount;
        }

        public long getMaxValue() { // Convenience method
            return maxValue(bitsPerValue);
        }

        public Object getArray() {
            return null;
        }

        public boolean hasArray() {
            return false;
        }
    }

    /** A write-once Writer.
     * @lucene.internal
     */
    public static abstract class Writer {
        protected final DataOutput out;
        protected final int bitsPerValue;
        protected final int valueCount;

        protected Writer(DataOutput out, int valueCount, int bitsPerValue) throws IOException {
            assert bitsPerValue <= 64;

            this.out = out;
            this.valueCount = valueCount;
            this.bitsPerValue = bitsPerValue;
            CodecUtil.writeHeader(out, CODEC_NAME, VERSION_CURRENT);
            out.writeVInt(bitsPerValue);
            out.writeVInt(valueCount);
        }

        public abstract void add(long v) throws IOException;

        public abstract void finish() throws IOException;
    }

    /**
     * Retrieve PackedInt data from the DataInput and return a packed int
     * structure based on it.
     * @param in positioned at the beginning of a stored packed int structure.
     * @return a read only random access capable array of positive integers.
     * @throws IOException if the structure could not be retrieved.
     * @lucene.internal
     */
    public static Reader getReader(DataInput in) throws IOException {
        CodecUtil.checkHeader(in, CODEC_NAME, VERSION_START, VERSION_START);
        final int bitsPerValue = in.readVInt();
        assert bitsPerValue > 0 && bitsPerValue <= 64 : "bitsPerValue=" + bitsPerValue;
        final int valueCount = in.readVInt();

        switch (bitsPerValue) {
            case 8:
                return new Direct8(in, valueCount);
            case 16:
                return new Direct16(in, valueCount);
            case 32:
                return new Direct32(in, valueCount);
            case 64:
                return new Direct64(in, valueCount);
            default:
                if (Constants.JRE_IS_64BIT || bitsPerValue >= 32) {
                    return new Packed64(in, valueCount, bitsPerValue);
                } else {
                    return new Packed32(in, valueCount, bitsPerValue);
                }
        }
    }

    /**
     * Create a packed integer array with the given amount of values initialized
     * to 0. the valueCount and the bitsPerValue cannot be changed after creation.
     * All Mutables known by this factory are kept fully in RAM.
     * @param valueCount   the number of elements.
     * @param bitsPerValue the number of bits available for any given value.
     * @return a mutable packed integer array.
     * @throws java.io.IOException if the Mutable could not be created. With the
     *         current implementations, this never happens, but the method
     *         signature allows for future persistence-backed Mutables.
     * @lucene.internal
     */
    public static Mutable getMutable(int valueCount, int bitsPerValue) {
        switch (bitsPerValue) {
            case 8:
                return new Direct8(valueCount);
            case 16:
                return new Direct16(valueCount);
            case 32:
                return new Direct32(valueCount);
            case 64:
                return new Direct64(valueCount);
            default:
                if (Constants.JRE_IS_64BIT || bitsPerValue >= 32) {
                    return new Packed64(valueCount, bitsPerValue);
                } else {
                    return new Packed32(valueCount, bitsPerValue);
                }
        }
    }

    /**
     * Create a packed integer array writer for the given number of values at the
     * given bits/value. Writers append to the given IndexOutput and has very
     * low memory overhead.
     * @param out          the destination for the produced bits.
     * @param valueCount   the number of elements.
     * @param bitsPerValue the number of bits available for any given value.
     * @return a Writer ready for receiving values.
     * @throws IOException if bits could not be written to out.
     * @lucene.internal
     */
    public static Writer getWriter(DataOutput out, int valueCount, int bitsPerValue) throws IOException {
        return new PackedWriter(out, valueCount, bitsPerValue);
    }

    /** Returns how many bits are required to hold values up
     *  to and including maxValue
     * @param maxValue the maximum value that should be representable.
     * @return the amount of bits needed to represent values from 0 to maxValue.
     * @lucene.internal
     */
    public static int bitsRequired(long maxValue) {
        // Very high long values does not translate well to double, so we do an
        // explicit check for the edge cases
        if (maxValue > 0x3FFFFFFFFFFFFFFFL) {
            return 63;
        }
        if (maxValue > 0x1FFFFFFFFFFFFFFFL) {
            return 62;
        }
        return Math.max(1, (int) Math.ceil(Math.log(1 + maxValue) / Math.log(2.0)));
    }

    /**
     * Calculates the maximum unsigned long that can be expressed with the given
     * number of bits.
     * @param bitsPerValue the number of bits available for any given value.
     * @return the maximum value for the given bits.
     * @lucene.internal
     */
    public static long maxValue(int bitsPerValue) {
        return bitsPerValue == 64 ? Long.MAX_VALUE : ~(~0L << bitsPerValue);
    }

    /** Rounds bitsPerValue up to 8, 16, 32 or 64. */
    public static int getNextFixedSize(int bitsPerValue) {
        if (bitsPerValue <= 8) {
            return 8;
        } else if (bitsPerValue <= 16) {
            return 16;
        } else if (bitsPerValue <= 32) {
            return 32;
        } else {
            return 64;
        }
    }

    /** Possibly wastes some storage in exchange for faster lookups */
    public static int getRoundedFixedSize(int bitsPerValue) {
        if (bitsPerValue > 58 || (bitsPerValue < 32 && bitsPerValue > 29)) { // 10% space-waste is ok
            return getNextFixedSize(bitsPerValue);
        } else {
            return bitsPerValue;
        }
    }
}
